Placenta
From Freepedia
Image:Human placenta baby side.jpg
The placenta is an ephemeral (temporary) organ present only in female placental mammals during gestation (pregnancy).
The placenta is composed of two parts, one of which, the chorion, is genetically and biologically part of the fetus, the other part of the mother. It is implanted in the wall of the uterus, where it receives nutrients from the mother's blood and passes out waste. This interface forms a barrier, the placental barrier, which filters out many substances which could harm the fetus. However, many other substances are not filtered out, including alcohol. Most viruses also easily cross this barrier.
In addition to the transfer of gases and nutrients, the placenta also has metabolic and endocrine activity. It produces, amongst other hormones, progesterone which is important in maintaining the pregnancy, and somatomammotropin (also known as Placental Lactogen) which acts to increase the amount of glucose and lipids in the maternal blood. This results in increased transfer of these nutrients to the fetus and is also the main cause of the increased blood sugar levels seen in pregnancy.
The placenta is connected to the fetus via the umbilical cord which is composed of blood vessels and connective tissue. When the fetus is delivered, the placenta is delivered afterwards (and for this reason is often called the afterbirth). After delivery of the placenta the umbilical cord is usually clamped and severed or may be left attached to fall off naturally which is referred to as a Lotus Birth. In most mammalian species, the mother bites the cord and consumes the placenta.
Some gynecologists in England claim that they have witnessed the practice of placenta consumption amongst humans in hospital wards in the 1990's (Lieberman, Dr. Telephone interview. University of London undergraduate assignment. 1999. London England).
The relationship between the mother and foetus is to some extent antagonistic. It is largely in the foetus's interest to draw more resources from the mother: the mother only shares half her genes with the foetus, so although they have a mutual interest in the success of the foetus, the antagonistic balance can be upset if, for example, the placenta over or under produces certain hormones.
The site of the former umbilical cord attachment in the center of the front of the abdomen is known as the umbilicus, or navel, or belly-button.
The only non-placental mammals are the monotremes, which are egg-laying mammals found only in Australia and New Guinea. Marsupials, which are found primarily in Australia, have a rudimentary egg-yolk placenta, which degenerates quite early in foetal development. Both marsupials and monotremes represent earlier branching of the basal mammalian lineage.
See also
| Endocrine system | |
| Adrenal gland - Corpus luteum - Hypothalamus - Ovaries - Pancreas - Parathyroid gland - Pineal gland - Pituitary gland - Testes - Thyroid gland |
